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Overlayfs (6.5+) accepts "::" in lowerdir to introduce data-only lower layers, but the option is split on ":" and the resulting empty element is treated as a path. Compaction then finds no common prefix and skips, so long options still fail with "mount options is too long", and an idmapped overlay mount fails with "no common prefix found". Skip the empty element when computing the common prefix and carry it through unchanged in both rewrites, so "::" survives; the idmapped rewrite would otherwise map it to the idmapped root and turn the data-only layers into regular lowers. Assisted-by: Claude:claude-opus-5-5 Signed-off-by: Mykhaylo Sul <mykhaylo.sul@oss.qualcomm.com>
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🟢 Approval recommended
No active review issues remain for the changes preserving Linux overlay data-only lower-layer separators.
No open issues 🎉
What changed in this PR
This PR updates Linux overlay mount handling to preserve :: data-only lower-layer separators, supporting snapshotters that return these options during ID mapping and long-option compaction.
Changes:
- Excludes empty separator elements from common-prefix calculations.
- Preserves separators when rewriting lower-layer paths.
- Adds regression tests for prefix calculation and both path rewrites.
| File | Description |
|---|---|
core/mount/mount_linux.go |
Filters separators from prefix calculations and preserves them in rewritten options. |
core/mount/mount_linux_test.go |
Adds data-only lower-layer regression cases for all three affected helpers. |
Review effort: Balanced
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Overlayfs (Linux 6.5+) supports
::data-only lowers. For snapshotters returning these mounts (none in-tree), treating the separator as a path breaks idmapped mounts ("no common prefix found") and long options ("mount options is too long").Exclude empty elements from prefix computation and preserve them in both rewrites.
Each added unit regression fails without the fix. A
mount.All()repro using :: with uidmap/gidmap or 150 lowers fails before and mounts after the fix on kernel 7.1.3.Developed with AI assistance; I reviewed and tested the change.